1. Field of the Invention
This invention concerns liquid crystal displays. Still more particularly, this invention concerns display systems in which a primary liquid crystal display becomes transparent in a failure mode and allows the viewer to observe a backup display located behind the primary LCD display when the primary display is disabled.
2. Description of the Prior Art
The last decade has witnessed a phenomenal increase in the use of digital as opposed to analog electronic instrumentation systems. In turn, liquid crystal display devices have been widely employed as display means for these digital electronic instrumentation systems. The liquid crystal display is highly desirable since it is a low power device which exhibits high visual contrast and it is adapted for a variety of different digital formats such as conventional alphanumeric and dot matrix displays. A few of the increasingly diverse uses to which liquid crystal displays have been put include aircraft flight control displays, automobile instrumentation, alphanumeric readouts for electronic calculators, wrist watches, electronic game displays, etc.
In particular, liquid crystal displays are being increasingly utilized as instrument displays in aircraft. It is particularly important in aircraft usages that redundant instrumentation be employed; hence, it is common for a primary instrument display to be presented in a central location while a redundant instrument supplying the same information will be necessarily located at some remote location within the cockpit of the aircraft. The problem, then, occurs when a failure occurs in the primary flight instrument display and the pilot is forced to look away from the centrally located primary display to view the corresponding backup instrument in a remote location. The disruption to the pilot's concentration in such circumstances can be quite severe and detrimental to the safe operation of the aircraft.
It is clear, therefore, that in aircraft, as well as many other liquid display usages, it would be highly desirable to be able to employ a primary liquid crystal display which would become transparent to the viewer, either upon power failure or the intentional disablement of the device, thereby enabling the viewer to observe other displays located directly behind the now transparent primary liquid crystal display. Unfortunately, no such fail transparent display system has been heretofore available. Conventional liquid crystal display technologies rely on either a mirrored reflected surface or a light trap behind the light blocking portions of the alphanumeric or graphic display of the liquid crystal display in order to provide sufficient contrast between the light blocking portions of the liquid crystal display activated by the electrical field and the background of the display for readability of the activated symbol.